In the past, it has been known to dip-form a dip-forming composition which contains a natural latex such as a latex of natural rubber to obtain a nipple, balloon, glove, medical balloon, sack, or other dip-formed article which is used in contact with the human body. However, a latex of natural rubber contains proteins which cause allergic reactions in the human body, so sometimes there was a problem as a dip-formed article which directly contacts the mucous membrane or organs of the body. Therefore, studies have been made on the use of a latex of a synthetic acrylonitrile-butadiene copolymer rubber or polyisoprene.
For example, Patent Document 1 discloses a dip-formed article which is obtained by dip-forming a dip-forming composition which comprises a latex of an acrylonitrile-butadiene copolymer rubber which contains ethylenically unsaturated acid monomer units in which sulfur and a vulcanization accelerator which comprises zinc oxide and a thiazole compound are mixed. The dip-formed article of Patent Document 1 does not contain any protein which can cause allergic reactions in the human body, but has a high stress at the time of 300% stretching and is not satisfactory in the point of flexibility.
Further, Patent Document 2 discloses a dip-forming composition which comprises a latex of synthetic polyisoprene in which sulfur, zinc oxide, a specific vulcanization accelerator, and a dispersant are mixed. The dip-forming composition which is disclosed in this Patent Document 2 can give a dip-formed article which is flexible and is excellent in tensile strength. In this Patent Document 2, an alkali earth metal salt of casein is used as a dispersant. Casein is one type of protein and is liable to cause allergic reactions in the human body.
Here, the dip-forming composition which contains a latex of synthetic polyisoprene is usually used for dip-forming after being aged for a suitable time period (also sometimes called “prevulcanization”) so as to maintain the tensile strength of the obtained dip-formed article stably high.
However, in the art of the above-mentioned Patent Document 2, when not mixing in an alkali earth metal salt of casein as a dispersant, the are the problem that unless the aging time is made longer, the obtained dip-formed article ends up becoming inferior in tensile strength and the problem that the latex of the synthetic polyisoprene ends up falling in dispersion stability and coarse coagulum form during aging resulting in use for dip-forming ending up becoming difficult.